Few of us are qualified medical professionals and given that this is regarding pregnancy and there could literally be a hundred different reasons why you're experiencing pain, it's best to call your doctor to see if they are at all concerned.

You could have a bladder infection, he could be hitting the bottom of the placenta which could cause pain, you could have a vaginal infection...really any number of things. Give the doc a call, I'm sure you're not the first woman to inquire about mysterious pains during sex, pregnancy, or sex while pregnant. At the very worst, they'll ask you to come in for a pelvic, urinalysis, and/or blood work.

I agree you should talk to your doctor. i had contraction and horrible stomach pains once while pregnant and it was just the round ligaments that were strained during sex. but it could be a million things. you should really get some medical opinions on it.